Tried the automatic upgrade again after restore of all files from the files backup: this time it worked as expected.
What was different:
There were 3 plugins (Akismet, All in one SEO pack, Reveal IDs) that required an update. So, after restoration of files from the backup, these 3 plugins have been updated first, before attempting the automatic upgrade again.
After that, the automatic upgrade worked (as usual) and Wp was successfully updated to v3.1, including the database upgrade.
Not sure what to make of that first failed attempt, perhaps is still worth a look.
